The invention concerns a device for determining a torque exerted on a shaft, wherein the shaft comprises a first shaft section and a second shaft section, the two shaft sections being rotatable relative to each other, and with a multi-pole magnetic ring which surrounds the first shaft section and is connected thereto, and a stator holder mounted to the second shaft section, wherein two stator elements are mounted to the stator holder, each stator element comprising fingers which project in an axially or radially outward direction, are uniformly distributed over at least part of the periphery, and are interspaced by gaps, wherein the fingers of one stator element and the fingers of the other stator element are associated with the magnetic ring.
The steering angle is conventionally detected by a code disc and an optical sensor scanning the code disc, wherein the optical sensor is stationary and the code disc is fixed to the steering shaft or vice versa.
The increased use of control systems for controlling the drive dynamics of vehicles has considerably increased the importance of steering angle sensors. These sensors generate a signal which characterizes the steering angle or the change in steering angle of a vehicle. Towards this end, an encoding element, in particular a code disc, is rigidly connected to the steering shaft. The code disc is associated with an optical sensor which is fixed relative to the chassis or steering shaft and which can read the code of the code disc. Markings are thereby disposed on the disc, e.g. in the form of lines or notches, which are scanned. The optical sensor may consist of a light diode (DE 199 36 245 A1), an optical fiber element and a scanning unit comprising several light receivers. The digital scanning principle is characterized by high reliability.
Further devices for measuring the steering angle are disclosed in DE 101 10 785 A1, DE 100 41 095 A1, DE 101 42 448 A1, WO 99 39 169 A1 which are optically operated, DE 199 41 464 A1, which is operated inductively, and DE 197 47 638 C1, DE 199 00 330 A1, DE 195 06 938 A1. DE 100 36 281 A1, WO 2002 071 019 A1 and DE 102 22 118 A1 which are operated magnetically. Optical methods are less suitable for application in the engine compartment, since higher temperatures, oil, fat, and dirt may disturb the optical system.
Devices of this type can determine the absolute steering angle, the steering direction, the steering speed and steering acceleration with relatively high precision, however, this data is insufficient e.g. for an electromechanical steering aid (EPAS). Moreover, the direction and size of the steering torque are required.
All conventional optical methods have the property that the accuracy of measurement of the differential angle between input and output shafts of a torsion piece, required to measure the torque, must be less than 0.05°. This is a problem for imaging optics, since the measurement of the position of structural edges must be much more precise due to direct optical imaging on the detector than the separation between the pixels of the optical sensor arrays. Measurement of the torque at both ends of a torsion piece using optical sensors is known e.g. from WO 99 09 385 A1. Scanning of a pattern on an encoder disc using optical means is disclosed e.g. in EP 0 777 851 A1.
The above-mentioned analog optical methods are less suitable for measuring small angles of rotation in steering systems for determining the torque, since they are less robust. As mentioned above, when the sensor is used on a steering gear in the engine compartment, there is the danger that the high-resolution analog optical system is disturbed by the prevailing higher temperatures and the presence of oil, fat and dust.
Optical sensors which are digitally operated with low resolution are suited for use in the engine compartment. Analog optical sensors with high resolution seem to be less suitable.
Magnetic methods for torque measurement are usually operated with magnetic multi-pole wheels and magnetoresistive sensors. The magnetic field sensor detects the relative position of the magnetic poles (DE 198 28 513 A1). This transforms the small mechanical turning angle into a large electric signal change to achieve high measuring precision.
DE 102 30 347 A1 discloses a device for measuring the steering angle and torque, which utilizes the magnetic measurement of the Hall sensor principle.
It is therefore the underlying purpose of the invention to further develop a device of the above-mentioned type in such a manner that the steering information required for ESP (electronic stability program) and EPS (electric power steering) can be reliably determined by one single device at little expense.